An active matrix substrate includes a switching element for each pixel, e.g., a thin film transistor (hereinafter, “TFT”). In the present specification, such a TFT will be referred to as a “pixel TFT”. As pixel TFTs, amorphous silicon TFTs whose active layers are an amorphous silicon film, and crystalline silicon TFTs whose active layers are a crystalline silicon film such as a polycrystalline silicon film, have been widely used.
In some cases, a part or a whole of a peripheral driving circuit may be integrally formed on the same substrate as the pixel TFTs. Such an active matrix substrate is referred to as a driver-monolithic active matrix substrate. In a driver-monolithic active matrix substrate, the peripheral driving circuit is to be provided in a region (a non-display region or a frame region) other than the region that contains a plurality of pixels (display region). The pixel TFTs and the TFTs composing the driving circuit (circuit TFTs) may be formed by using the same semiconductor film. As this semiconductor film, for example, a polycrystalline silicon film with a high field-effect mobility may be used.
Moreover, it has been proposed to use an oxide semiconductor as the material of the active layers of TFTs, instead of an amorphous silicon or a polycrystalline silicon. It has also been proposed to use, as an oxide semiconductor, an In—Ga—Zn—O type semiconductor whose main components are indium, gallium, zinc, and oxygen. These TFTs are called “oxide semiconductor TFTs”. An oxide semiconductor provides a higher mobility than does an amorphous silicon. Therefore, oxide semiconductor TFTs can operate more rapidly than amorphous silicon TFTs. Moreover, an oxide semiconductor film is formed through a simple process as compared to a polycrystalline silicon film, and therefore is applicable to devices which require a large geometric area. Therefore, by using an oxide semiconductor film, it also becomes possible to form pixel TFTs and circuit TFTs integrally on the same substrate.
However, regardless of whether a polycrystalline silicon film or an oxide semiconductor film is used, it is difficult to adequately satisfy the characteristics that are required of both pixel TFTs and circuit TFTs.
On the other hand, Patent Document 1 discloses an active-matrix type liquid crystal panel including oxide semiconductor TFTs as the pixel TFTs and TFTs whose active layers are a non-oxide semiconductor film (e.g., crystalline silicon TFTs) as the circuit TFTs. In the liquid crystal panel of Patent Document 1, the oxide semiconductor TFTs and the crystalline silicon TFTs are formed on the same substrate. Patent Document 1 states that use of oxide semiconductor TFTs as the pixel TFTs can reduce display unevenness, and that use of crystalline silicon TFTs as the circuit TFTs enables rapidly driving.